This invention relates to the casting of metal strip. It has particular but not exclusive application to the casting of ferrous metal strip.
It is known to cast metal strip by continuous casting in a twin roll caster. Molten metal is introduced between a pair of counter-rotated horizontal casting rolls which are cooled so that metal shells solidify on the moving casting roll surfaces and are brought together at the nip between the casting rolls to produce a solidified strip product delivered downwardly from the nip. The term “nip” is used herein to refer to the general region at which the casting rolls are closest together. The molten metal may be poured from a ladle into a smaller vessel or series of smaller vessels from which it flows through a metal delivery nozzle located above the nip, so forming a casting pool of molten metal supported on the casting roll surfaces of the rolls immediately above the nip. This casting pool may be confined between side confining plates or dams held in sliding engagement adjacent the ends of the casting rolls.
Although, twin roll casting has been applied with some success to non-ferrous metals which solidify rapidly on cooling, there have been problems in applying the technique to the casting of ferrous metals which have high solidification temperatures and tend to produce defects in the cast strip caused by uneven solidification on the casting roll surfaces of the rolls. One particular problem arises from the formation of pieces of solid metal known as “skulls” in the casting pool in the region of the side confining plates. These problems are exacerbated when efforts are made to reduce the superheat of the incoming molten metal. The rate of heat loss from the melt pool is greatest near the confining plates due primarily to additional conductive heat transfer through the confining plates and the roll ends. This high rate of local heat loss is reflected in the tendency to form “skulls” of solid metal in this region which can grow to a considerable size and go through the nip between the rolls causing defects in the strip generally known as “snake eggs”. It is therefore very important to maintain constant pool conditions in the casting pool in the region of the side confining plates.
We have found that the distance between the nearest nozzle ends of in the delivery nozzle and the inner faces of the confining side plates is particularly important to inhibit formation of “skulls” in the casting pool in this region. We have determined that significant flow changes are brought about by variation in this distance. Variation in this distance may be brought about by inaccurate location of the confining plates or the delivery nozzle during set up, or by subsequent change in the distance due to thermal expansion and wear in the confining plates or the nozzle openings of the delivery nozzles during casting. This problem remains even if the delivery nozzle is designed specifically to provide an increased flow of metal to the “triple point” regions (i.e., where the confining plates and casting rolls meet at the meniscus regions of the casting pool) and increase the heat input to these regions of the casting pool. Examples of such nozzles may be seen in U.S. Pat. Nos. 4,694,887, 5,221,511 and our earlier Australian Patent Application 35218/97 based on provisional Application P02367.
Although triple point pouring has been effective to reduce the formation of skulls in the triple point regions of the casting pool, it has not been possible completely to eliminate the problem. The generation of skulls and resulting strip defects has been found to be remarkably sensitive to even minor variations in the flow of metal into the triple point regions of the casting pool. Even minor changes in the distance between the nozzle ends (where the nearest nozzle openings are located) and the confining plates due to thermal expansion and/or wear has been found to be sufficient to cause defects in the strip. As the distances between the nozzle ends and the confining plates are reduced the downwardly inclined flow of metal from the triple point pouring passages in the ends of the nozzle impinges higher on the confining plates. This change can lead to the formation of skulls in the casting pool and subsequent snake egg defects in the strip. In extreme cases, changes in these distances can cause the poured molten metal to surge upwardly between the nozzle ends and confining plates, and spill over the upper edges of the confining plates.
This problem is addressed in our Australian Patent Application 63175/99 which discloses an improvement by which it is possible to maintain substantially constant spacing between the ends of the delivery nozzle and the confining plates with wear of the confining plates during the casting campaign, and which sets forth embodiments of the present invention. In Application 63175/99, there is disclosed apparatus for casting metal strip comprising:
a pair of casting rolls forming a nip between them,
an elongate metal delivery nozzle formed in a plurality of discrete elongate nozzle pieces disposed end to end,
nozzle supports supporting the nozzle pieces such that the delivery nozzle extends above and along the nip between the casting rolls for delivery of molten metal to form a casting pool of molten metal supported on casting surfaces of the casting rolls above the nip,
a pair of pool confining plates adjacent the ends of the nip,
plate biases to bias the pool confining plates adjacent the ends of the casting surfaces of the casting rolls so that the confining plates move inwardly along the rolls to accommodate wear of the confining plates, and
nozzle end shifters to shift the nozzle pieces having outer nozzle ends nearest the confining plates on the nozzle supports with inward movements matching the inward movements of said confining plates accommodating wear of the confining plates to maintain substantially constant spacings between the confining plates and the nearest nozzle ends.
In the specific apparatus disclosed in Australian provisional application PP8024, the nozzle end shifter comprises spacers disposed between the nearest outer nozzle ends and the side confining plates to set the spacings between said nozzle ends and the side plates so that the confining plates through the spacers push the ends of the delivery nozzle inwardly as the confining plates move inwardly under the influence of the biasing force to accommodate wear of the confining plates. This disclosure is also set forth in provisional application PQ0071, from which the present application claims priority.
An alternative apparatus also disclosed in Australian provisional application PQ0071 provides for a nozzle shifter to shift the outer nozzle ends to provide more reliable control of the distance between the confining side plates and the nearest outer nozzle ends during the casting campaign. This alternative apparatus for casting metal strip comprises:
a pair of casting rolls forming a nip between them,
an elongate metal delivery nozzle formed in a plurality of discrete elongate pieces disposed end to end along the nip,
nozzle supports supporting the nozzle pieces such that the delivery nozzle extends above the nip to discharge molten metal through the nozzle pieces to form a casting pool of molten metal supported by the casting rolls above the nip,
a pair of pool confining plates adjacent the ends of the nip to confine the casting pool,
plate biases to bias the pool confining plates adjacent end surfaces of the casting rolls to move the confining plates inwardly to accommodate wear of the plates, and
nozzle end shifters to shift the nozzle pieces defining the outer nozzle ends of the delivery nozzle with inward movements matching the inward movements of said side plates accommodating wear of the side plates to maintain substantially constant spacings between the side plates and the nozzle ends, wherein the nozzle end shifters comprise a pair of moveable structures disposed one at each end of the casting roll assembly, drives to move the moveable structures longitudinally of the rolls, nozzle attachments to attach the moveable structures to the two nozzle pieces defining the outer nozzle ends nearest the confining plates so that those two nozzle pieces are moved with the movable structures, and controls responsive to inward advances of the confining plates along the casting rolls to cause the drives to move the moveable structures inwardly and shift said two nozzle pieces with inward movements matching inward movement of the confining plates.
The plate biases may comprise a pair of generally horizontally acting thrusters actuable to apply opposing inward closure forces to the confining plates. Said moveable structures may provide abutments against which the thrusters react to apply the inward moving forces to the confining plates.
The moveable structures may comprise a pair of carriages which carry the thrusters and which are moveable toward and away from another to enable the spacing between them to be adjusted so that the carriages can be preset before a casting operation to suit the width of the casting rolls.
The moveable structures may further comprise carriage drives acting between outer end parts of the moveable structures and the carriages to move the carriages toward and away from one another.
The carriage drives may comprise a pair of fluid operable cylinder units connected one to each of the carriages and to outer end parts of said moveable structures.
The drives may act on the outer end parts of the moveable structures.
The drives may comprise a pair of jacks connected to the outer end parts of the moveable structures. Those jacks may be electrically driven screw operated jacks.
The controls may be responsive to motion of the plate biases which produces inward movements of the pool confining plates. The controls may, for example, include transducers in the plate thrusters to produce control signals indicative of movement of the thrusters and plates and connected in a control circuit with the drives such that the drives cause corresponding movements of the moveable structures and therefore said two nozzle pieces.
Alternatively the controls may include inspectors, such a sensors or video cameras, to observe the position of the pool confining plates and to provide control signals dependant on observed changes in the position of those plates.
In addition, broadly disclosed is an apparatus for casting metal strip comprising:
(a) a pair of casting rolls forming a nip there between;
(b) a pair of confining plates adjacent the ends of the casting rolls;
(c) an elongated metal delivery nozzle having a plurality of discrete nozzle pieces disposed along the nip capable of discharging molten metal to form a casting pool supported on the casting rolls above the nip confined by the confining plates;
(d) nozzle supports capable of supporting the nozzle pieces defining outer nozzle ends of the delivery nozzle nearest the confining plates; and
(e) delivery nozzle drives capable of moving the nozzle pieces defining said outer nozzle ends to control the distance between said outer nozzle ends and said confining plates.
The nozzle ends in the nozzle pieces nearest the confining plates have nozzle openings to deliver molten metal to the triple point region of the casting pool.
The nozzle pieces are moved by the nozzle drives with or along the nozzle supports depending on the embodiment. In either event, the delivery nozzle drives may vary the distance between the confining plates and the outer nozzle ends nearest the confining plates to maintain appropriate flow of molten metal into the triple point region while allowing for thermal expansion and wear of the nozzle pieces and confining plates and inhibit formation of skulls in the casting pool. The apparatus may also comprise an inspector, such as a video camera, to allow an operator to monitor the melt flow in the triple point region and electrical controls actuated by an operator may energize the nozzle drives to move the nozzles pieces relative to the confining plates.
The apparatus for casting metal strip may have the delivery nozzle drive capable of moving the nozzle ends nearest the confining plates to maintain a set distance between the outer nozzle ends and the confining plates with thermal expansion and wear of the confining plates, the nozzle ends or both. The apparatus for casting metal strip may have said set distance set and maintained on the order of 15 millimeters and less, and may be between about 7 and 9 millimeters. The apparatus may further comprise inspectors, such sensors or video cameras, to sense or observe the distance of the confining plates from the outer nozzle ends, and provide electrical signals, automatically or by an operator, to the delivery nozzle drives to maintain the distance between the confining plates and the outer nozzle ends with thermal expansion and wear or the confining plates or the outer nozzle ends, or both.
The broad apparatus may also comprise biases to force the confining plates inwardly adjacent the ends of casting rolls. The biases may be separate from the nozzle drives to allow the distance between the outer nozzle ends-and the confining plates to be varied separate from the movement of the confining plates, or the biases may be provided with the nozzle drives if the distance between the outer nozzle ends and the confining plates are to be maintained at a set distance. Alternatively, the biases may be provided by a separate drive such as a servo mechanism.
Also broadly disclosed is a method for casting metal strip comprising the steps of:
(a) assembling a pair of casting rolls to form a nip between the casting rolls and a pair of confining plates adjacent the ends of the casting rolls pool,
(b) assembling an elongated metal delivery nozzle with a plurality of nozzle pieces disposed along the nip capable of discharging molten metal to form a casting pool supported on the casting rolls above the nip confined by the confining plates, and
(c) moving the nozzle pieces defining the outer nozzle ends of the delivery nozzle to control the distance between the confining plates and said outer nozzle ends. The method may vary the distance between the confining plates and the outer nozzle ends nearest the confining plates to maintain appropriate flow of molten metal into the triple point region while allowing for thermal expansion and wear of the nozzle pieces and confining plates, and inhibiting formation of skulls in the casting pool. The method may also comprise inspecting the casting pool in the triple point region, as with a video camera, to allow an operator to monitor the melt flow in that region and control movement of the nozzle drives and in turn the nozzles pieces defining the outer nozzle ends relative to the confining plates.
Alternatively, the method for casting metal strip may include the step of moving the nozzle pieces defining the outer nozzle end nearest the confining plates to maintain a set distance between the nozzle openings and the confining plates with thermal expansion and wear of the confining plates, the nozzle ends or both. The method of casting metal strip may include setting the distance between the confining plates and the nearest nozzle ends on the order of 15 millimeters or less and may be between about 7 and 9 millimeters, and maintaining said set distance during the casting campaign. The method may also comprise the further step of inspecting the distance of the confining plates from the outer nozzle ends, and providing electrical signals to control circuits of the delivery nozzle drives to maintain the set distance between the confining plates and the outer nozzle ends with wear and thermal expansion of the confining plates or the outer nozzle ends, or both.
The method may also comprise the step of biasing the confining plate inwardly adjacent the end of the casting rolls. This step may be done separately from moving the nozzle pieces defining the outer end of the delivery nozzle, or may be done along with the step of moving said nozzle pieces if the distance between the confining plates and the outer nozzle ends is to be set and maintained during the casting campaign.